Current trends in complementary metal oxide semiconductor (CMOS) technology have led to improved digital circuit performance often at the expense of analog circuit performance. The difficulty of designing high performance analog circuits is clearly apparent when designing mixed signal products fabricated using a logic-based advanced process. State-of-art transistors optimized for digital applications use super halo (or pocket) implants to control short channel effects. Some effects of using super halo implants are increasing transistor threshold voltage mismatch, as well as increased output conductance, which both of which degrade analog circuit performance. In addition, several high performance analog circuits, such as cascode-based charge pumps and current mirrors, require very low threshold voltages for proper performance. As CMOS technology continues to advance, halo doses are expected to continue to increase, further exacerbating the difficulties of maintaining usable analog circuit performance.
In large part, analog circuit performance degradation is caused by transistor threshold voltage mismatch and high output conductance in advanced CMOS technologies. The gate of a transistor controls the channel charge with only a relatively minor portion of the channel charge controlled by the drain. The threshold voltage of a transistor is mainly determined by the gate controlled portion of the channel charge. The channel charge controlled by the drain becomes more significant when the doping of the drain of the transistor is very high relative to the doping of the channel. In addition, the random doping fluctuation of the well will affect the drain depletion region, which is a more significant effect in a lightly doped channel. High output conductance in current CMOS technology is a result of a large difference in doping levels between pocket and channel doping. A large imbalance of relative doping between the pocket implant doping level and the channel doping level leads to a reduction of the output conductance, especially for long channel devices. The high pocket implant doping level relative to the channel doping level causes saturation of the drain current at a lower drain bias in regions of the transistor with the pocket doping level compared to regions of the transistor with the channel doping level.
As a result, there is a need to solve the problems of the prior art to provide a method and apparatus for improving analog circuit performance in mixed signal processes.